1. Field of the Invention
The present invention relates to a three-valued switching circuit having one input terminal and one output terminal to obtain output with three kinds of current values from one output terminal.
2. Description of the Related Art
With reference to FIG. 17, a conventional current switching circuit will be described. In FIG. 17, the current switching circuit includes two current sources 92 and 93 connected to a current mode circuit 91, and switches SW1 and SW2 for switching these current sources 92 and 93. The current sources 92 and 93 have current values of IO and IO/2, respectively. To a control input terminal 94, a low level or high level control input signal is applied. This control input signal is supplied, as a switching signal of the switches SW1 and SW2, through an inverter 95 to the switch SW2 and further through an inverter 96 to the switch SW1.
The current switching circuit is structured to have two current sources to be switched by the switches SW1 and SW2 for attaining different electrical characteristics by changing a current value of the current mode circuit 91. More specifically, one of the two current sources 92 and 93 can be selected by selecting the switch SW1 or the switch SW2 in response to one control input signal at a logical high or low level.
The above-described current switching circuit, however, requires current sources and switches to be prepared as many as the number of current values to be varied.
On the other hand, a three-valued logic circuit enabling one current source to have three kinds of output is known (e.g. Utility Model Laying-Open No. 2-6684). In the three-valued logic circuit, output is allotted to one of three output terminals according to three kinds of potentials applied to an input terminal and three output terminals are accordingly needed. Output currents of the three output terminals are the same.